1. Field of the Invention
The present invention relates to a stage for specifying a shape of an irradiation surface onto which a laser beam is irradiated. Further, the present invention relates to a laser irradiation apparatus in which energy distribution of a laser beam is made uniform over a certain specific region. The present invention also relates to a method of making an energy distribution uniform, and to an optical system to realize the uniformity. Furthermore, the present invention relates to a method of making the energy distribution of a laser beam uniform over a certain specified region, and to a method of annealing a semiconductor film using the laser beam (the method is hereafter referred to as “laser annealing”). The present invention also relates to a method of manufacturing a semiconductor device, the semiconductor device having circuits structured by thin film transistors (hereafter referred to as “TFTs”), which includes a laser annealing process. For example, electro-optical devices, typically liquid crystal display devices, and an electronic equipment in which such an electro-optical device is mounted as a part, are all included in the category of the semiconductor devices. Namely, the term “semiconductor device” as used throughout the specification indicates general devices capable of functioning by utilizing semiconductor device characteristics, and electro-optical devices, semiconductor circuits, and electronics all fall under the category of semiconductor devices.
2. Description of the Related Art
Techniques for performing crystallization, or for increasing crystallinity, by performing laser annealing of amorphous semiconductor films or crystalline semiconductor films (semiconductor films that are not single crystal, but have crystallinity such as poly-crystallinity or micro-crystallinity), in other words non-single crystal semiconductor films formed on an insulating substrate such as glass have been undergoing widespread research recently. Films such as silicon films are often used as the semiconductor films.
Glass substrate are low cost, and can be easily made into large surface area substrates, compared with conventional quartz substrates, which are often used. This is because the above research is actively performed. Lasers can impart a high energy to only a non-single crystal semiconductor film, without causing much change in the temperature of a substrate, and therefore lasers are suitable for annealing semiconductor films formed on the glass substrates having low melting point temperature (the distortion temperature of generally available glass substrate is on the order of 600° C.).
Crystalline semiconductor films formed by laser annealing have high mobility. The manufacture of TFTs on a single glass substrate, used for driving pixels and used in driver circuits, by using the crystalline semiconductor films is therefore flourishing, along with the manufacture of active matrix liquid crystal electro-optical devices. The crystalline semiconductor films are made from many crystal grains, and therefore they are also referred to as polycrystalline semiconductor films.
In laser annealing, a method in which a laser beam of a pulse oscillation type excimer laser or the like having high output is formed by an optical system so as to have a square shape spot of several cm/square, or a linear shape with a length equal to or greater than 10 cm, on an irradiation surface, and then scanning of the laser beam is performed (the irradiation position of the laser beam is made to move relative to the irradiation surface) is preferably used because it has good productivity and is industrially superior.
In particular, if a linear shape laser beam (hereafter referred to as a linear shape beam) is used, then the laser beam can be irradiated over the entire irradiation surface by scanning only in a direction perpendicular to the longitudinal direction of the linear shape beam. This differs from a case of using a spot shape laser bean, with which it is necessary to scan forward and backward, left and right, and therefore high productivity can be obtained. Scanning in a direction perpendicular to the longitudinal direction is performed because that scanning direction has the best efficiency. Due to their high efficiency, linear shape beams formed by appropriate optical systems are being used mainly in laser annealing processes. Note that, within this specification, the direction of the long side of the linear shape beam is referred to as a longitudinal direction, while the short side is referred to as a transverse direction.
An example of an optical system for forming the shape of a laser beam into a linear shape on an irradiation surface is illustrated. The optical system shown in FIG. 2 is an extremely general one. The optical system not only converts the shape of the laser beam into a linear shape on the irradiation surface, but at the same time it makes the energy distribution of the laser beam uniform. In general, optical systems for making the beam energy distribution more uniform are referred to as beam homogenizers. The optical system shown in FIG. 2 is one of beam homogenizers.
Synthetic quartz may be used in all cases, for example, as the base material of the optical light system, provided that an excimer laser to be an ultraviolet light is used as a light source. This is true because a high transmissivity can be obtained. Further, there may be employed a coating material capable of obtaining a transmissivity of 99% or more with respect to the wavelength of the excimer laser used as a coating.
The side view of FIG. 2 is explained first. A plane containing the light axis and parallel to the page of the side view is taken as a meridional plane, and a plane containing the light axis and perpendicular to the meridional plane is taken as a sagittal plane. The direction of the light axis changes here for cases in which it is necessary to bend the light path by using mirrors or the like due to the layout of the optical system, and it is assumed that the meridional plane and the sagittal plane also change at this time. A laser beam output from a laser oscillator 1201 is divided in a direction perpendicular to the sagittal plane by cylindrical lens arrays 1202a and 1202b. With this structure, there are four cylindrical lenses contained in the cylindrical lens array 1201, and therefore four divisions are made. It is assumed that the number of cylindrical lenses contained in the cylindrical lens array 1202b is also four. The divided laser beams are made to mutually overlap in a certain plane by a cylindrical lens 1204. It is not always necessary to use the two cylindrical lens arrays 1202a and 1202b; one cylindrical lens array may also be used. The advantages of using two cylindrical lens arrays are that the size of the linear shape beam can be changed, and that the width of the linear shape beam in the transverse direction can be made shorter.
The once again divided laser beams are bent at a right angle by a mirror 1207, and then made to once again overlap on an irradiation surface 1209 by using a doublet cylindrical lens 1208. The doublet cylindrical lens designates a lens structured by two cylindrical lenses. Uniformity of the energy distribution is thus formed in the transverse direction of the linear shape beam, and the width in the transverse direction of the linear shape beam is determined. The mirror 1207 is used in order to make the irradiation surface into a level surface, and is not always necessary.
The upper view of FIG. 2 is explained next. The laser beam emitted from the laser oscillator 1201 is divided in a direction perpendicular to the meridional plane by the cylindrical lens array 1203. There are seven lenses contained in the cylindrical lens array 1203 with this structure, and therefore the laser beam is divided into seven divisions. Two cylindrical lens arrays 1203 may also be used in order to change the length of the linear shape beam in the longitudinal direction. The laser beams are then made to overlap into one beam on the irradiation surface 1209 by a cylindrical lens 1205. Shown from the mirror 1207 forward by dashed lines are the correct light paths, and the positions of the lens and the irradiation surface, for a case in which the mirror 1207 is not disposed. Uniformity of the energy distribution in the longitudinal direction of the linear shape beam is thus formed, and the length of the linear shape beam in the longitudinal direction is determined.
The length L of the linear shape beam is determined by the following elements: a width d of the cylindrical lenses contained in the cylindrical lens array 1203; a focal length f1 of the cylindrical lenses; and a focal length f2 of the cylindrical lens 1205. This is explained while following FIG. 3. A cylindrical lens 1301 is structured by cylindrical lenses having a width d. A laser beam made incident to the cylindrical lens array 1301 is condensed into a plurality of positions by the location of the focal length f1. The laser beam then becomes incident to a cylindrical lens 1302 while expanding. The cylindrical lens 1302 is a convex lens, and therefore two parallel light fluxes within the figure are each concentrated at positions located at a distance f2 behind the cylindrical lens 1302. The distance f2 is the focal length of the cylindrical lens 1302. The laser beams that are each incident to the cylindrical lens array 1301 are thus converted to a linear shape beam having a length L. A simple calculation shows that:L=d·f2/f1  (1)[Eq. 1]As explained above, the cylindrical lens array 1202a, the cylindrical lens array 1202b, and the cylindrical lens array 1203 function as lenses that divide the laser beam. The obtained uniformity of the laser beams is determined by the number of divisions. With the aforementioned structure, there are four divisions by seven divisions, and therefore a total of 28 divisions are formed.
By irradiating the linear shape beam thus formed in accordance with the above structure so as to be overlapped while gradually shifting the beam in the transverse direction, laser annealing can be performed with respect to the entire surface of a non-single crystal silicon film, for example, and the crystallization can be achieved and the crystallinity of the film can be increased.
The shape of laser beams emitted from excimer lasers is generally rectangular, falling with an aspect ratio range of approximately one to five. The laser beam strength shows a Gaussian distribution in which it becomes stronger toward the center. The size of the laser beam can be converted, for example, into a uniform energy distribution 300 mm×0.4 mm linear shape beam by the optical system shown in FIG. 2.
According to an experiment performed by the applicants of the present invention, the overlap pitch is most suitably set to approximately 1/10th of the width of the linear shape beam in the transverse direction when irradiating a linear shape beam of pulse oscillation with respect to a semiconductor film. Namely, if the width in the transverse direction of the linear shape beam is 0.4 mm, then laser annealing may be performed while shifting the semiconductor film in the transverse direction of the linear shape beam by 0.04 mm during the time from one pulse of light is emitted until the next pulse of light is emitted. The uniformity of laser annealing of the semiconductor film is thus increased. The method discussed thus far is an extremely general method used in order to perform laser annealing of a semiconductor film by using a linear shape beam.
The increasingly high output of laser oscillators has been remarkable recently, and laser oscillators capable of having a linear shape beam length exceeding 300 mm have become available. However, the substrate size used in production plants has also changed, and 600 mm×720 mm substrates, and 1000×1200 mm substrate are now planned, for example. A length on the order of 300 mm for the linear shape beam is becoming insufficient. If a case of annealing a semiconductor film formed on a 600 mm×720 mm substrate, for example, using a 300 mm long linear shape beam is considered, the longitudinal direction of the linear shape beam and the short side (the side having a length of 600 mm) of the substrate may be disposed in parallel. Half of the substrate surface can be laser annealed by scanning relatively the substrate with respect to the linear shape beam over a distance of 720 mm with in the direction of the long side of the substrate. Provided that the remaining half surface then undergoes laser annealing by the similar method, the entire surface of the substrate can be laser annealed.
There is exerted a bad influence on the throughput and on the footprint if laser annealing is performed by this type of method, namely scanning of the substrate (or the linear shape beam) must be performed two or more times, and the substrate (or the linear shape beam) must be moved forward and backward, and left and right. Furthermore, if laser annealing is performed on one half of the substrate at the time using the linear shape beam, then one half of the semiconductor film is laser annealed uniformly. However, uniformity is lost in the vicinity of the boundary between the one and the other half of the semiconductor film, and it is difficult to form semiconductor elements in this portion. Looking at these problem points, it becomes clear that it is preferable to make the length of the linear shape beam in the longitudinal direction at least on the same order as the length of the short side of the substrate.
However, there is a tendency for the path length of the optical system to become longer in order to form a long linear shape beam. For example, an optical path length on the order of 5000 mm is necessary to make a 300 mm long linear shape beam, further the optical path length will exceed 10,000 mm in order to make a 1000 mm long linear shape beam.